[PJL]

You can expect hum when running on a lamp limiter.

You need to get it on full power before you can determine if you still have problems. I will leave Techman to give detailed advice:
1. Replace any remaining capacitors.
2. Measure the resistance of R45 and R46 which should be 22 ohms, and the common cathode bypass resistor R44 which should be 600. If R44 is less than 600 ohms then it would be best replaced.
3. Check output stage current by measuring common cathode bypass voltage (I=V/R). It should be 52V according to ERT but much less with the limiter.
4. Take out the lamp limiter.
5. Monitor HT on first switch on. It should rise to 400+V then fall back to around 375V.
6. Check common cathode bypass volts again to confirm current is still to specification. It should now be 52V but switch off if above 55V.
7. Measure voltage across R45 and R46 which should be slightly less than 1V. Now calculate the anode current from the resistance measurements taken above (I=V/R). It is important that the valves are fairly well matched when using common cathode bias.

Not sure if you have checked the resistance values but there is a very good chance that many will have changed value and would be best replaced. The first stage anode resistor, R27, would be better as 2W metal film as it will introduce less noise.

The ERT indicates the PX4 anode dissipation = (365-52-1)*.043 = 13.4W. The spec for the later PX4's was 15W.